The present invention pertains to a method for sensing the phase transformation of a liquid into a solid and vice versa More particularly, the invention pertains to a method for determining when a solid phase of a material has formed at a location submerged in the material, which method depends upon the fact that the electrical resistance of the liquid and solid phases of a substance differ. Although the present invention is effective for monitoring any substance whose solid phase has an electrical resistance different from the electrical resistance of the liquid phase (believed to be true of virtually all substances), it is primarily directed for monitoring phase changes in water to be used in connection with water cooling systems; the following discussion, accordingly, concentrates on characteristics of such an application.
A common method for determining the progression of ice submerged in liquid water has been to monitor the changing resistance of the water. Because the electrical resistance of ice, solid phase water, is much higher than that of liquid water, such monitoring has enabled fairly accurate determination of the presence of ice. However, numerous problems can arise which can cause, previous methods to provide inaccurate indications. In certain situations, these problems can make successful application of the sensing method impossible.
Variations of this common method are essential in cooling systems such as those which employ a compressor and circulating refrigerant to maintain an ice pack within liquid water, which water might be used directly or circulated about an object for cooling the object. In such systems, the sensing method is part of closed-loop feedback control system which causes the compressor to operate often enough to maintain the water at its freezing point but not often enough to cause excessive ice formation. Since the compressor either operates or is shut off, such a control scheme is usually referred to as an "on-off" or "bank-bang" controller. In such systems, an error signal which causes the compressor to shut off is derived by monitoring the resistance of the water surrounding the cooling coils, which coils may carry refrigerant and thereby remove heat from the water The resistance of the water is then compared with a reference value and, if there is a difference, an error signal is produced In the prior art, the reference value is ordinarily a fixed, standard value believed to correspond to that of liquid water. Usually, while the cooling system is cooling the liquid, the liquid crystalizes progressively and radially outward from the cooling coils. The method is employed to interrupt the cooling cycle when the ice progresses to a predetermined position. The resistance of the water used to produce the error signal, hereinafter called the variable resistance, is maintained at the predetermined position. When the value of the variable resistance rises above the reference value, the previous methods indicate that ice has formed at the predetermined position, and this indication initiates the interruption of the cooling process.
A fundamental characteristic of all on-off controllers is oscillation of the controlled variable about the set point. Since the actuator of such a system is operated in an on-off fashion, environmental influences will cause the controlled variable to deviate from the set point when the actuator is off. This produces an error signal to turn the actuator on until the error signal is reduced to zero, and the cycle repeats. Such cycling is undesirable, however, when it is rapid enough, and the actuator is a mechanical device such as a compressor. Rapid start-stop cycles ("fast-cycling") cause excessive wear to the compressor as well as inefficient use of energy. A well known technique of solving this problem is to incorporate a "dead-band" into the controller. A "dead-band" is a range about which the controlled variable is allowed to deviate from the set point before the actuator is either activated or turned off. This is accomplished by making the set point of the system vary between two values according to whether the actuator is on or off. Thus, when the controlled variable is between the higher and lower set points, there is no change in the previously derived error signal which causes the actuator to either remain on or off. Therefore, the range between the higher and lower set points is effectively a "dead-band." In water cooling applications, precise control of the controlled variable is not necessary since the only objective of the control scheme is to prevent excessive ice formation around the cooling coils. Incorporating a dead-band into the system, therefore, involves no significant disadvantages.
The most basic of previous similar methods for controlling water cooling involves, simply, measuring the electrical resistance between a single probe and a grounded reference. Circuitry or other means are connected to the probe in order to measure this resistance and compare it with a predetermined fixed value of resistance, which value has been previously determined as a standard for water. A fundamental problem with such a method utilizing a single resistance reading is that there is no means for providing a dead-band. Lack of a dead-band causes the afore-mentioned practical problems-the compressor may undergo rapid start-stop cycles ("fast-cycling") when the progression of ice is immediately adjacent the probe. To solve this problem of fast-cycling, a dead-band must be incorporated into the control system by either mechanical or electronic means.
Another previous method involves monitoring resistance sensors from two probes. The utilization of two probes in this latter method effectively provides for a dead-band. A dead-band is achieved by electronically requiring that both probes sense the ice in order to stop the compressor while also requiring that the ice melt off both probes in order to activate the compressor once again. A predetermined value for the resistance of water has invariably been used as a reference value for this and each other of the previous methods. This previous method involves positioning a first of the two probes nearer the cooling coils so that it will ordinarily sense the progression of ice before the second probe and will sense melting after the second probe. The dead-band, therefore, occurs when the first probe senses the ice and when the ice melts from around the second probe. When the system is in this state, the compressor will remain in its previous operating mode, either on or off. If the total volume of the ice pack surrounding the cooling coils is viewed as the controlled variable, this system will cause that variable to oscillate between two set points, represented by ice surrounding the first and both probes, respectively.
Unfortunately, since each of the previous inventions depends on a fixed, predetermined value for the resistance of liquid water, the resulting indications are not always accurate since extraneous and nonstandard factors affect the resistance readings within any tank of water, particularly after extended usage. Most basically, the resistance of water may vary in different geographical locations due to local impurities in the water, which impurities generally raise the resistance of water. Resistance of the liquid water may similarly change within a system over time due to evaporation of the water, which evaporation raises the amount of impurities per volume of water remaining. Increased resistance of liquid water is also caused by increased amounts of impurities within the system due to accumulation over time. Employment of similar methods also creates problems in systems where the impurity content of the liquid water or the identity of the liquid is purposefully altered; in such situations the reference value must be changed, thus causing delays, particularly when circuitry must be accordingly modified.
Furthermore, deposits on submerged electrical probes, which deposits are natural over time, often affect the resistance measurements. The additional resistance of deposited impurities adds to the resistance which the probe reads, thereby raising the apparent resistance of the water. With the passage of time, coatings of such impurities inevitably adhere to virtually any probe which is submerged in liquid water that is the slightest bit impure. Notably, these coatings tend to be of uniform thickness on surfaces that are subjected to similar environments. Electrolytical plating on the probes may also affect the apparent resistance of the water as recorded by such probes. The electrolytical type deposits have been minimized with some previous methods by utilizing an alternating current rather than a direct current; however, in practice, slight electrolytical plating still occurs with an alternating current. Electrical probes necessary with every employment of the art, therefore, must be periodically replaced or cleaned when incorporated for use with previous inventions.
Therefore, it is a primary object of the present invention to account for resistance variances of water as well as apparent variances in this resistance caused by impurities deposited on the electrical probes, while providing a method for indicating the phase transformation of liquid water to ice.
Furthermore, previous methods have utilized a grounded electrical reference that is dependent on the system in which the previous method is employed. This presents a particular problem where the method is employed in a container or system that is insulated. It is, therefore, another object of the present invention to incorporate the use of a ground probe which is independent from the system in which the method is employed.
It is another object of the present invention to effectively minimize electrolytical plating and coating of electrical probes utilized with the invention.
It is also an object of the present invention to provide an apparatus which utilizes and enables the method of the present invention.
Additionally, it is an object of the present invention to provide a method for sensing the presence of a solid phase of any material within a liquid phase of any material, including but not limited to water.
It is another object of the present invention to avoid fast-cycling of apparatuses related to any particular employment of the present invention by providing for a dead-band.
These and other objects and advantages will be clear to those skilled in the art who have the benefit of this disclosure from the following detailed description of a preferred embodiment of the present invention.